Plating aluminum



1953 E. R. CALDERON ET AL 2,654,701

PLATING ALUMINUM Filed June 8, 1950 I DEGEEAsEI '1 ETC Era BLEACH Aer/1447's Acr/vArE LP/NSE L 5 J7 Auay Ste/K5 .Eownv 4P. Clubs-Q0,

GEOEGE W 620/141,

INVENTORS.

BY zw Patented Oct. 6, 1953 UNITED STATES 'ATENT OFFICE PLATIN G ALUMINUM Application June 8, 1950, Serial No. 166,839

16 Claims.

. This invention relates to improvements in the art of plating on the surfaces of objects made of aluminum and alloys thereof and is also applicable to the treatment of castings from zincbase alloys. The invention is particularly directed to a plating process in which the use of dangerous and poisonous acid pickles and etching solutions is eliminated, and the surfaces to be treated are provided with the uniform, adherent coating or plating which will not develop the incipient blistering which characterizes prior attempts to plate aluminum and aluminum alloys in extruded, drawn or cast form. The invention also contemplates the use of a novel strike solution which results in the formation of a tenaciously adherent, fine grained alloy plating which forms a base for subsequent electroplating with copper, nickle and other metals.

The process of this invention is applicable not only to commercially pure aluminum but is also applicable to castings, die castings, wrought alloys, etc. These various aluminum alloys may contain silicon, copper, magnesium, manganese and chromium, either alone or in combination. In the past, the same solutions for etching, brightening, etc. could not be used on pure aluminum and on the wrought or cast aluminum objects containing alloying components. The socalled zinc-immersion process (Patent No. 1,627,900) has been used to some extent in the past but this process is complicated, requires separate handling for various types of alloys and is not satisfactory, plated objects exhibiting pitting and latent blistering; that is, the plated part which apears to be free of blisters when first plated begins to develop small blisters within a few weeks, these blisters continuing to grow in size and making the part useless for outdoor exposure applications or wherever perfect finish is desired. It is to be remembered that plating is not only of value in changing the color or appearance of the article, imparting higher brightness and reflectivity thereto, but is also of considerable value in increasing the resistance of the object to abrasion or wear, eliminating a tendency toward smudging which is common to many aluminum alloys, and is also of value in facilitating soft solder joining, improving electrical contacts or conductivity, and increasing resistance to corrosion. stances final coating with brass is extremely desirable in order to allow vulcanization and adherence of rubber to the plated aluminum part.

The zinc-immersion process is not only subject to a very high percentage of rejects due to In some inthe pitting and latent blistering mentioned hereinabove but is also troublesome in that it is difiicult to obtain uniform plating on large or irregularly shaped articles. In addition, the process contemplates the use of mixed acid solutions, such as solutions of nitric and hydrofluoric acids or of mixtures of chromic acid and sulfuric acid. These pickles, etches and solutions are hazardous, toxic, poisonous and in many instances can only be successfully retained in costly carbon brick-lined and lead tanks, and in addition, require very efficient ventilating facilities in order to reduce the hazards of use.

The present invention is directed to a method of procedure with solutions to be described hereinafter in greater detail, the procedure being much more simple than any of the procedures contemplated and recommended heretofore. Substantially the same steps are employed, in accordance with the present process, on objects made from substantially pure aluminum and on objects made from aluminum alloys high in silicon, copper, manganese, etc. For example, the etching solutions employed difier but slightly in concentration and temperature of use; the remaining solutions including the strike solution are identical, some variation in temperature of utilization of the activating solution being the only difference between the treatment of objects made of virtually pure aluminum and those made from aluminum alloys. In all instances standard black iron or steel tanks may be used for the etching solutions, activating solutions and strike solutions.

The invention contemplates the control of pH and surface tension of many of the solutions employed in the process and a strike solution containing cyanides of'copper, silver, gold, zinc and indium, whereby an alloy containing silver or gold is deposited upon the surface of the object, the resulting alloy film being characterized by a fine grained structure which is sub= stantially non-porous.

An object of the present invention therefore is to disclose and provide a simple and economical process of plating objects of aluminum and aluminum alloys.

A further object of the invention is to disclose and provide an improved process of plating aluminum, said process eliminating the necessity of using dangerous or toxic mixtures of nitric and hydrofluoric acid or mixtures of chromic and sulphuric acid.

Another object of the invention is to disclose and provide a plating process, particularly appliits alloys, which virtually for expensive tanks and cable to aluminum and eliminates the necessity ventilation equipment.

A still further and important object of the present invention is to disclose and provide a process of plating aluminum and its alloys, said process insuring uniformity, homogeneity, fine grain structure practically free from porosity and tenacious adherence to the surface of the aluminum object.

An object is to disclose and provide a plating process whereby a ternary alloy is electrodeposited upon the surface of aluminum objects, said alloy including silver in diffused form through;- out such ternary alloy.

A still further object of the invention is to disclose and provide a method ofplating zincbase die-casting alloys with a ternary alloy strike solution in order to increase the corrosion resistance of the die-casting alloys. Furthermore, it will be apparent from the following description, an object of the invention is to disclose and provide stable solutions and the use of solutions inhibited for excessive solubility of copper and silicon therein.

These and various objects, uses and advantages of the present invention will become apparent, to those familiar with the practice recommended in the art heretofore, from a contemplation of the description given hereinafter. In orderto facilitate understanding reference will be held to the appended chart or diagram illustrating the sequence of operations which is preferably followed in the plating of aluminum and aluminum alloys.

As will be readily seen from the contemplation of the appended diagram, the objects to be plated are first degreased, as indicated at iii, and then passed to an etching bath. Etching baths are indicated at HA and HB, since the temperature and concentration of these etching solutions varies somewhat as to whether relatively pure aluminum objects are being treated or objects made from alloys. The path of relatively pure aluminum objects is indicated by theheavy line whereas the path of the objects composed of aluminum alloys containing silicon, manganese, copper, etc. is indicated by the lighter line pass.- ing through HB. After the etching operation, both types of objects are rinsed at 12, then bleached at l3, rinsed at M and then passed through an activating solution at ISA and IB. Although the activating solution is the same in both instances certain differences in temperature of immersion are preferably followed as will become apparent from the subsequent description. During such activation a thin adherent film of zinc is formed on the bleached surface of the ob jects. The objects are then rinsed at It and are subjected to an alloy strike at H, whereby an alloy characterized by fine grain and lack of porosity is formed on the surface of the objects. The objects are then rinsed, as indicated at l8, and may then pass to a relatively normal copper plating step, as indicated at i9, or if desired, to a subsequent plating indicated at 20, whereby nickel, chromium, cadmium, silver, gold, etc. may be plated upon the objects by any standard process. Steps l9 and 20 are indicated in dash lines since they do not form a part of the present invention. The degreasing operation is also indicated in dash lines at In for th same reason.

Although vapor degreasing with tetrachloroethylene is recommended, the use of inhibited alkaline cleaners incapable of exerting an etching effect upon the aluminum is not prohibited. Marking inks and other identification marks should be removed. Ietrachloroethylene is recommended as a solvent for degreasing aluminum since it is more stable than other chlorinated solvents. The acidity of the solvent should be checked at regular intervals and a degreaser should be thoroughly cleaned out periodically in accordance with careful plant. practice.

In accordance with the present process, the etching solutions employed at HA and B are relatively mild solutions containing the alkalih-ydroxides and alkali-fluorides. Either sodium orpotassium hydroxide and fluoride may be used. Purified grades of these chemicals are preferred and technical or commercial grades are not recommended.

It may be noted at this time that in order to insure freedom from blisters, pinholes and other defects, it is very desirable that all of the solutions, including the etching solution, bleaching solution, activating solution and strike solution be made with either distilled water or deionized water which may be obtained by treatment of ordinary tap water with base exchange resins as is well known in the art.

The following table indicates the composition of the etching solutions used at HA and B. It will be noted that for commercially pure aluminimum or aluminum alloys with a low content of alloying components from A of an come to 2 ounces of sodium fluoride is used per gallon of the etching solution. Thefigure in parentheses'indie cates the preferred proportion of sodium fluoride.

C 11A 11B figgg g Aluminum Alloys Sodium Hydroxide 3 o z./gal 3 oglgal, sodiurn Fluoride (l)2 oz./gal 2.0 (2.5)4 oz./gal.

Although the etching solution is made of the same components the preferred temperature at which such solution is kept during immersion varies and the time of immersion of the object in such. solution varies. Intank HA the solution is preferably kept at a temperature of 130 *5. and the objects are immersed 15' to 45 seconds. In etching operation IIB the etching solution is maintained at anoperating temperature of about lflQ" If. and theobjectsare immersed for a period of'eto 60 seconds. After such immersion under theconditions stated the objects are subjected to a thorough rinse, as indicated at l2.

It may be noted at this point that the etching solutions will attack the surface of the objects without attacking the underlying grain structure. A slightly mat finish is observed.

The objects then pass through a bleaching bath, indicatedat 13;. The primary purpose of the bleaching bath isto brighten the surface of the objects and remove any copper which may have been precipitated. During etching, alloys low in copper content will not be effected appreciably but alloys containing appreciable amounts of copper and magnesium will acquire a film whieh is dark and this must be fairly and completely bleached off in the nitric acid solution. The bleaching solution consists of a nitric acid solution preferably maintained at a temperature of about F. or F. Commercial nitric acid may be diluted with distilled or deionized water, the concentration of the nitric acid solution not being critical. One, two or even three volumes of nitric acidmay be added to one part of water. All of the objects being treated should be clipped in this solution for a period of not less than seconds. Ordinarily the relatively pure aluminum objects may be dipped into the solution for a period of 15 to 45 seconds, this solution being maintained at temperatures of between about 80 F. and 90 F. Aluminum alloys high in copper or silicon may be dipped for a period of from 45 to as long as 60 seconds and the solution is preferably maintained at a temperature of between about 95 F. and 105 F. Every trace of nitric acid must be rinsed off before proceeding to the next step and such rinsing is indicated at M.

The thoroughly clean surface of the objects being treated is then subjected to activation as indicated at USA and I5B. Such activation is attained by treating the surface with an aqueous alkaline solution of potassium zincate. Preferably this solution contains a surface tension reducing agent or depressant of the alkaline type whereby the surface tension of the solution is reduced to below 50 dynes per square centimeter and preferably to a range of between about and dynes. Moreover, the activating solution preferably contains an agent adapted to suppress the solution of silicon and copper and zinc sulphate heptahydrate, potassium selenate, potassium tellurate and zinc phosphate are ex amples of agents which may be successfully used.

The potassium zincate contained in this solution may be prepared in various manners, for example, by the reaction of purified zinc oxide or other zinc compound with molten potassium hydroxide or potassium monoxide; by the dissolution of pure electrolytic zinc in aqueous solutions of potassium hydroxide; by the electrochemical dissolution of zinc in potassium hydroxide employing an alternating current of frequency of to cycles across the electrodes of electrolytic zinc, or in the following manner: in 82 ounces of potassium hydroxide, dissolved in a minimum quantity of water, there is dissolved 10 ounces of zinc oxide. Separately v6 ounces of zinc sulphate heptahydrate is dissolved in a min- I imum quantity of hot water to which there has been added 6 ounces of potassium hydroxide in the form of a saturated solution. The two solutions are then mixed together and diluted to one gallon with distilled water. The surface tension depressant is now added and the prepared solution is filtered before use. The mixing of the solution repressing agent in caustic potash solution with the solution of potassium zincate, followed by filtration is the preferred procedure.

Preferably, the activating solution contains 85-90 oz. of KOI-I and 9-11 oz. of ZnO per gallon. Caustic soda and sodium zincate is not recommended since pinholes, incomplete films and 0cclusions which form blisters follow the use of sodium compounds. It is believed that since sodium carbonate is readily formed and is relatively insoluble in the caustic solution, such carbonates precipitate and act as nuclea which engender blisters. Potassium carbonate, even if formed, is five times as soluble, and it has been found that the recommended solutions do not leave microscopically small particles nor bare spots on the activated surface.

It may be noted that the activating solution should be free from chlorides and nitrates since it has been found that these elements are highly detrimental. Furthermore, attention is again called to the preferred temperature ranges since the combination of optimum reaction temperature, low surface tension and presence of the solubility depressing agent insures instantaneous wetting of the article to be plated and instantaneous coverage with zinc. Not only is the coverage with zinc rapid but it is uniform and homogeneous, thereby insuring proper and uniform, blister-free adherence of the subsequently applied ternary alloy strike.

After the operations described hereinabove, the object, composed essentially of aluminum, is rinsed at I5 and subjected to the action of an alloy strike solution at H. Heretofore, after the normal zinc-immersion in a sodium zincate solution, the object Was subjected to a Rochelle type copper cyanide strike. The present process distinguishes therefrom in excluding sodium compounds from the step preceding a strike, and in employing a strike solution containing zinc, copper and a metal from the group consisting of silver, gold and indium (preferably in the form of cyanides), under conditions which insure the rapid electrodeposition of a ternary alloy on the surface of the object. The ternary alloy strike solution of the present invention is distinctive not only because of the presence of three compatible metals which form an alloy, but also by reason of the fact that the solution is stabilized, buifered, has a pH of between 9 and 10 and a low surface tension. The pH range is critical; if the pI-I is below 9, there is a tendency to decompose the cyanides present; if the pH is above 10, there is a decided tendency to dissolve zinc. Solutions adjusted to this desired range (by the controlled addition of tartaric acid or potassium hydroxide) are extremely stable and have changed but 0.2 pH in a year. The surface tension of the solution is preferably adjusted to below about 40 dynes per square centimeter by the use of a surface tension depressant of the alkaline type.

During a strike, large quantities of hydrogen are evolved, and the low surface tension of the strike of this invention results in immediate liberation of such hydrogen, perfect wetting of the surface, and the formation of a continuous, homogeneous and uniform film of alloy over the entire surface, even though the object is large and has irregularly shaped surfaces. Effective surface tension reducing agents can be selected from those which are primarily aliphatic alcohols, and are stable in alkaline solutions: cyclic animes or betaines containing one non-cyclic hydrocarbon radical of 10 to 20 carbon atoms, are examples of satisfactory agents.

The use of ammonia as a brightener has been eliminated and, instead, monoethanolamine or thiethanolamine is preferably employed, these materials having a concurrent surface tension reducing effect.

A typical ternary alloy strike solution, prepared in accordance with this invention, comprises:

Potassium cuprocyanide, 4 oz. per gal.

Silver cyanide, 0.12-(0.25)-0.35 oz. per gal.

Zn cyanide (in equivalent combining quantity of KCN) 0.8-(1.5)-1.8 oz. per gal.

Monoethanolamine, 25-(38)-45 ml. per gal.

Surface tension depressant, 5-(10)-15 ml. per

After the above illustrative components are dissolved in warm water, a saturated solution of tartaric acid may be slowly added to adjust the pH to the range of between 9 and 10 (preferably between 9.2 and 9.8) and the solution filtered.

amma 1. pl-l should' be determined by: a: glass. electrode.-.

This ternary strike solution can be usedv in. black iron or steel-tanks; thetankssacting aszthe. anode; The .solution shouldlbe. ata temperature:- of about 105 F. when at work. Strike the work with current on beiorexsubmergingthet work. Immersion times may vary iromz;-sec.-- onds to as long as 90 seconds; depending-upon the morphology of; the. work .beingplated. rent densities on the order; of:1thos.e. usually; emeployed in a: strike (say 241 amp. per sq. it) can be used initially and. then reduced. Some circulation or agitation ofthesolution' isidesirable; and the solufion should be: filtered regularly-in: order to insure .a homogeneous, .non-porous:.plate;.

Generally stated, the molecular ratios'oii. the.- metals in: this strike: should; be. maintained. at between aboutv 3. to 5 Cuzand 0;l5;-0:3i-.Ag-;toi -1; of Zn; When indium is substitutedufor silverronly 0.1-0.2Inneedbezused;

Attention is again calledto theiact that. no sodium compounds, chlorides. or nitrates-should be inthe strike- The film, resulting from. strike solution. above described, bonded. to the surface of: the object. and is. of very fine grain structure, the silver; gold or: indium being: diffiised; throughout the, grain. boundaries;

After the strike and plating with;v a. ternary alloy as described, .the: object b.e platedwith. any otherm'etalin accordance withknownprccesses. For example, if; it is desired to. copper plate, as indicated. at li9.;the inventors-preterm; plate with a hot, strongly alkalinehighqspeed: copper solution, using temperatures of; 180? since. no injuryto theialuminumiobject results; Potassium cuprocyanide. solutions: are; desirable; with the use. of. periodic, cyclic. reverse current, flow, since speed, consistency, perfect. covering; and high. corrosion resistance areattained;

As previously stated, the ternary strike solu tion may be used onzincebase'castingsjwith great successbecause: the controlled pH precludes'solu tion or; attack of the zinc and the combination of components andpropertiesresults;inzaphighly effective protective; coating of: exceptional den?- sity.

By the use of. the process, conditions;and-111a.-- terials disclosed herein, objects. composed; essenr the use of: the. nos/e1:

tially: of aluminum (this phrase-including both.

relatively pure aluminum; and its: alloys) can be; plated, effectively with. less than 2%; or 5..%,.- oft rejects. Objects plated by: this invention; have. not shown blistering with age-and have .successfully withstood 72v hours of. an accelerated salt. spray test.

All changes, adaptations, uses. and; modifications coming within the scope of the appended.- claims are embraced: thereby.

We claim:

1. A method of providing objects, composed; essentially of aluminum, with a tenaciously adherent, fine grained alloy plating, comprising? etching the degreased 'surface of: objects com-- posed essentially of aluminum" with an aqueous solution of alkali hydroxide and alkali fluoride and rinsing the" etched surface; then treating the surface with diluted'nitric' acidand rinsing the surface; then forming a thin, adherent'fi'l'm of zincron such treatedsurface;,-rinsing theactivated surface and electroplating -the: activated. surface with an aqueousstrike solution-contain ing cyanides of copper, zinc: and-.ta...metal from the group consisting of silver, gold and indium,

' perfectl while: imposing an? electrical; current; upon. the

of'fbetweerrabout.IOOPF; and 110. F.

4; A .methodof the character stated in claim 1' whereinthe strike solution is virtually free/from sodium compounds, chlorides, andj' nitrates and:

is at a temperature of between about F. and 1130.?

'5... A, method; of providing: objects. composed;

essentially of aluminum, with a. tenaciously, art--- herent, fine; grained. alloy plating, comprising: etching. the. degreased surfaceof, objects: compos'edihessentially of: aluminum with: an. aqueous. solution of alkali hydroxide and alkali fluorideat a; temperature oi: between. about 125 F. and- 145" FL, said-solution containing from about 0.75 oz. to; 4102; of sodium fluoride per gallon, and

rinsingthe etchedsurface; then. treating the.

surface with diluted nitric. acid. and rinsing the. surface; then forming a thin, adherent film of zinc on such-treated s-urfaceaby immersion. in. an aqueous, alkali-nesolution of potassium zincate containing asolution. depressant for silicon and: copper; and. a surface; tension: depressant, at a temperatureof about; 80 F. and F.; rinsing; the-acti atedsuri ace and-electroplating the activatedsurface in an. aqueous strike solution containing; cyanides. of. copper, zinc and a metal from: the group consisting of silver, gold and; indium, said strike solutionhaving a. pH of. between '9; and 10 and. asurfacetension below 45 dynes'persquare centimeten.

6. A method of the-character stated in. claim 5 wherein. thestrike solution is virtually freefrom. sodium compounds, chlorides and nitrates;

'7. A-niethod of thecharacter stated in. claim 5.- wherein the strike solution is at atemperatureof between about 100 F.-andr" 8; A method of thercharacter'stated in.=claim 5: wherein thestrikasolution. is. virtually free from: sodium compounds; chlorides and nitrates. and is-ata temperature-of-between aboutlflO? F.. andll.0. E.

9. A. method: of, the character; stated in. claim. 5.wherein.the= solution depressantior silicon and. copper. is .zinc, sulphate.

10. A method of-the character. stated in claim 5 wherein the aqueous, alkaline solutionoi potas-.-- sium: zincateis virtually free from sodium compounds, .chlorides and nitrates, and-has a surface.v tension below 50 dynes per'square. centimeter.

1.1.. In the-plating of objects consisting essentiallyof aluminum provided with a zinc surface. film and zinc-base? castings, the. formation of an .alloy strike :deposit uponthesuriaces of such: objects and castings, from; an aqueous strike. so.- lution having; a. pH of between-9 and 10 and: a surface. tension; of below l5 'clynes per square.

centimeter; while imposing an electrical current upon the objects, said solution containing cyanides. of copper, zinc and a. metal. from they group consisting ofa-silver, goldand indium.

12. A process of. the character stated in claim 11, wherein the strike solution contains potassium cyanide and potassium hydroxide and is virtually free fromsodium compounds, chlorides and nitrates- 13; Apro'cess-of the character stated in claim 11 wherein the strike solution is maintained at a temperature of between about 100 F. and 110 F.

14. A process of the character stated in claim 11 wherein the solution contains monoethanolamine as a brightener.

15. A method of providing objects, composed essentially of aluminum, with a tenaciously adherent, fine grained alloy plating, comprising: etching the degreased surface of objects composed essentially of aluminum with an aqueous solution of alkali and alkali fluoride and rinsing the etched surface; then treating the surface with diluted nitric acid and rinsin the surface; then forming a thin adherent film of zinc on such treated surface by immersion in an aqueous potassium zincate bath virtually free from sodium compounds; rinsing the activated surface and electroplating the activated surface in an aqueous strike solution containing cyanides of copper, zinc and a metal from the group consisting of silver, gold and indium, said strike solution having a pH of between 9 and 10 and being virtually free from sodium compounds, chlorides and nitrates.

10 16. A method of the character stated in claim 1 wherein the etch solution contains sodium hydroxide and from about .75 oz. to 4.0 oz. sodium fluoride per gallon.

5 EDWIN R. CALDERO-N.

GEORGE W. SLOMIN.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,104,842 Smith July 28, 1914 1,627,900 Hewitson May 10, 1927 1,923,790 Moore Aug. 22, 1933 1,938,961 Gravell Dec. 12, 1933 1,943,853 Austin Jan. 16, 1934 1,954,353 Ernst Apr. 10, 1934 2,142,564 Korpiun Jan. 3, 1939 2,151,302 Scheller Mar. 21, 1939 2,496,845 Balden Feb. 7, 1950 2,501,737 Porter et al Mar. 28, 1950 2,513,365 Rogofi July 4, 1950 OTHER REFERENCES Metal Finishing, August 1940, page 435. 

1. A METHOD OF PROVIDING OBJECTS, COMPOSED ESSENTIALLY OF ALUMINUM, WITH A TENACIOUSLY ADHERENT, FINE GRAINED ALLOY PLATING, COMPRISING: ETCHING THE DEGREASED SURFACE OF OBJECTS COMPOSED ESSENTIALLY OF ALUMINUM WITH AN AQUEOUS SOLUTION OF ALKALI HYDROXIDE AND ALKALI FLUORIDE AND RINSING THE ETCHED SURFACE; THEN TREATING THE SURFACE WITH DILUTED NITRIC ACID AND RINSING THE SURFACE; THEN FORMING A THIN, ADHERENT FILM OF ZINC ON SUCH TREATED SURFACE; RINSING THE ACTIVATED SURFACE AND ELECTROPLATING THE ACTIVATED SURFACE WITH AN AQUEOUS STRIKE SOLUTION CONTAINING CYANIDES OF COPPER, ZINC AND A METAL FROM THE GROUP CONSISTING OF SILVER, GOLD AND INDIUM, WHILE IMPOSING AN ELECTRICAL CURRENT UPON THE OJBECTS, SAID STRIKE SOLUTION HAVING A PH OF BETWEEN 9 AND 10 AND A SURFACE TENSION BELOW 45 DYNES PER SQUARE CENTIMETER. 